Sheets of a rigid polymeric material, such as polyvinyl chloride ("pvc") or blends of pvc and other polymers, such as an acrylic resin, make excellent wall coverings, especially for places where walls are subject to abuse and damage from objects rolling or carried about the space, a common situation in hospitals, schools, and various heavily used public spaces. The assignee of the present invention has for many years marketed a blend of pvc and an acrylic resin in rigid sheet form for use as a wall covering as part of its line of "Acrovyn.RTM." wall protection products. The "Acrovyn.RTM." wall covering sheets are highly resistant to marring and breakage from impact, are easy to clean and are supplied in a wide range of colors. One disadvantage of "Acrovyn.RTM." wall covering sheets, however, is the difficulty of installing them. For one thing, the underlying wall must be reasonably smooth and in good condition, inasmuch as irregularities in the wall "telegraph" through the "Acrovyn.RTM." sheets. Accordingly, when the wall to be covered is one having an irregular surface, for example concrete block, brick or ceramic tile, it is necessary to install a smooth wall over the rough one, the most common practice being to install drywall over a tile, block or brick wall. Where "Acrovyn.RTM." sheets are used in renovation, the wall must be carefully prepared ---- for example, plaster and drywall have to be patched and spackled to eliminate cracks, dents and other imperfections or damaged spots.
"Acrovyn.RTM." sheets are adhered to the wall by a water-based contact cement (e.g., 3M "Fastbond 30"). Ordinarily, a first coat of cement is applied to the wall and allowed to dry. Then, a second coat is applied to the wall and also allowed to dry. Finally, a coat of cement is applied to the sheet and allowed to partially dry until it is tacky. With the aid of slip sheet, the panel is placed in proper position. The slip sheet is progressively removed from between the wall and the panel and at the same time the panel is pressed against the wall. Getting the sheet into proper position requires considerable skill, and there is no second chance, because the contact adhesive sticks on contact and does not allow the panel to be reset or slid into proper position. The cement gives off a noxious vapor, which requires that the work area be well-ventilated and that the space where the installation is going on and nearby spaces be unoccupied. Although the contact cement is water-based, clean-up after installation requires a solvent. Despite the foregoing difficulties, "Acrovyn.RTM." and similar rigid sheets of polymeric materials are popular and widely used, because they are attractive, highly durable and easy to care for. Because they are quite thick and are of uniform color throughout, scratches and other moderate damage is not very noticeable. Commonly, the polymer sheets have a textured front face, which not only makes them look good but helps hide scratches. The availability of a variety of impact protection products (corner guards, bumper guards, handrails, etc.) of the same or a similar polymeric material enhances the desirability of the rigid polymeric material wall covering from the point of view of affording to architects and designers the opportunity to create modern, clean-looking interior designs that will stand up to years of abuse.
Wall panel systems based on panels of drywall, "Masonite" or some other rigid substrate sheet covered with thin pvc sheets (typically 0.006 inch thick) are available. Such systems are usually installed using a construction adhesive over an underlying wall and include moldings at all joints between panels and between the panels and elements of the building that they abut, such as ceilings, doors, windows and the like. Universally, the moldings extend over the front faces of the panels adjacent their edges, which diminishes their appearance, makes them harder to keep clean, especially near the moldings, and in the case of vertical moldings presents a projection that can be struck by objects carried or rolled through the space. The thin pvc sheet is not very durable -- it is prone to tearing when struck.
A problem that has heretofore not been solved effectively is how to provide a wall panel system based on rigid sheets of polymeric material adhered to a substrate sheet that can be installed with conventional construction adhesives over an irregular wall or a wall that is in bad condition, thereby eliminating the need for extensive and costly preparation of the wall. One aspect of that problem is preventing warping of the panels due to changes in ambient humidity or to dampness of the underlying wall. If the panel tends to warp under such conditions, it will almost certainly become at least partly detached from the underlying wall, because industrial adhesives do not have the tenacity to prevent the panel from pulling away from the underlying wall, if the panel should warp. A further complication is the necessity for such a wall panel system to conform to fire codes. It is also desirable to eliminate vertical moldings that project from the plane of the front faces of the panels.